This invention relates to methods for use in recovery systems wherein one or more adsorber vessels are used to adsorb chemical substances from a carrier medium, and live steam, for example, is the desorption medium. Solvents are typically the chemical substances to be adsorbed and recovered, and activated carbon is commonly used as the adsorbent.
In many industries, including the petrochemical, pharmaceutical, and printing industries, heavy use of solvents is made in a variety of activities. In many of these activities, solvents which are highly volatile may evaporate to the surrounding carrier medium which may be air or some other gas. This loss of solvents is undesirable for several reasons. Because solvents are expensive, their loss represents an additional operating cost of the user facility. Moreover, many evaporated solvents that are discharged into the atmosphere are pollutants owing to their odor, unsightliness, or danger to the environment. Thus, it is common for the solvent laden air or gas to be passed through a solvent recovery system, whereby the solvent is extracted by adsorption from the air or gas for reuse.
Recovery of a chemical substance in such a recovery system is not limited to solvents, however. Any chemical substance that may be adsorbed on an adsorbent medium can be recovered. The explanation of the present invention and its background will be in terms of solvent recovery systems for the sake of simplicity, but it is understood that such explanation is equally applicable to recovery systems for nonsolvent chemical substances that are adsorbable.
During the adsorption stage of a conventional recovery process, solvent laden gas is passed through an adsorber vessel, wherein a bed of adsorbent extracts the solvent from the gas by adsorption. The cleansed gas is then expelled from the adsorber; in the case where the gas is simply air, it can be expelled directly into the atmosphere. Just before the activated carbon becomes saturated with solvent and begins to allow undesirable amounts of solvent to pass, the adsorption vessel is removed from adsorption service, and the desorption or "scrubbing" stage commences.
During desorption, wherein steam will hereinafter be used as the example of a suitable desorption medium, live steam is passed through the adsorbent in the direction opposite to the flow of solvent laden gas during the adsorption stage. After the steam sufficiently heats the adsorbent containing the solvent, the solvent begins to be desorbed. At first, the solvent is desorbed at a relatively high rate, but as the amount of solvent in the adsorbent decreases, the rate of desorption falls and a disproportionately large amount of steam is used to desorb the remaining solvent. Continued scrubbing with live steam to remove the last amount of solvent would be prohibitively expensive, so as a practical matter the scrubbing stage must be terminated short of complete desorption. After termination of the desorption stage, the adsorber vessel containing the adsorbent must be cooled and dryed so that the adsorber can again be placed in adsorption service.
A typical solvent recovery system will include at least two adsorber vessels for cyclic operation to permit continuous adsorption by the adsorbers, one may be operated for adsorption service while the other is being scrubbed; then, when the first adsorber must be scrubbed, the latter one can be returned to adsorption service.